Process for manufacturing ferritic hot rolled steel strip

ABSTRACT

A process for manufacturing a ferritic hot rolled steel strip is provided. The process includes providing a steel slab, hot rolling the slab to produce a transfer bar and ferritic hot rolling the transfer bar to produce hot rolled strip. The ferritic hot rolled strip is coiled at temperatures between 580-780° C. and has a yield strength between 130-210 MPa, a tensile strength greater than 260 MPa, a uniform elongation greater than 15%, a total elongation to failure greater than 30%, and an n-value greater than 0.2.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 61/728,554 filed Nov. 20, 2012, which is incorporated in its entirety herein by reference.

FIELD OF THE INVENTION

The present invention relates in general to a process for manufacturing ferritic hot rolled steel strip, and in particular to a process for producing ferritic hot rolled carbon steel that can be used for components via deep draw stamping.

BACKGROUND OF THE INVENTION

Processing of low carbon steels, e.g. steels with 0.02-0.10 wt % carbon (C), to produce hot rolled sheet, strip, etc. that can be further processed via stamping, drawing, cold rolling, and the like is known. Typically, such hot rolled strip is produced by hot rolling of the steel in the austenite phase region of the material (austenitic rolling) close to the transformation temperature Ar3 in order to receive a normalized evenly distributed fine grained structure after the transformation following the rolling process.

It is also known that ultra low carbon (ULC <0.005 wt % C) and extra low carbon (ELC: <0.02 wt % C) steels, including interstitial free steels with a reduced manganese content (<0.3 wt % Mn), have been subjected to “ferritic rolling” in which the steel is rolled in the ferrite region in or during the last stands of a hot rolling finishing mill and thereby producing a coarser grain structure compared to LC steels. Ferritic rolling of ULC and ELC steels can produce desirable soft and formable steel grades, however it can also produce a high risk of uncontrolled rolling behavior and cobbling in a finishing train. Heretofore methods have attempted to solve this problem by replacing one of the rolling stands with an additional water cooling step during finish rolling, which requires investment and alteration of a hot strip mill finishing train.

Given the above, an improved process for ferritic rolling of LC carbon steels is desired in which neither alteration of the finishing stand configuration nor interstand cooling is necessary. In addition, a process where steels with higher carbon contents than the ELC grades that can be used to produce soft bake hardening (BH) steels with a minimum yield strength between 180 and 210 megapascals and a sufficient shelf time capability after a hot dip galvanizing process or a continuous annealing process without an over-aging section would be desirable.

SUMMARY OF THE INVENTION

A process for manufacturing a ferritic hot rolled steel strip is provided. The process includes providing a steel slab with a chemical composition in weight percent (wt %) within a range of 0.10 maximum (max) carbon (C), 0.15-0.60 manganese (Mn), 0.20 max silicon (Si), 0.04 max titanium (Ti), 0.008 max vanadium (V), 0.006 max molybdenum (Mo), 0.1 max nickel (Ni), 0.05 max chromium (Cr), 0.08 max copper (Cu), 0.015 max sulfur (S), 0.04 max phosphorus (P), 0.01 max nitrogen (N), 0.006 max boron (B), 0.06 max aluminum (Al), balance iron (Fe) and incidental melting impurities known to those skilled in the art.

A steel slab having a chemical composition within the above-stated range is soaked at temperatures between 1100-1400° C. and then hot rolled using a roughing treatment at temperatures between 900-1400° C. in order to produce a transfer bar. The transfer bar is hot rolled using a finishing treatment with finishing treatment entry temperatures between 900-1100° C. and finishing treatment exit temperatures between 720-850° C. The finishing treatment produces hot rolled strip which is coiled at a coiling station at temperatures between 580-780° C. The hot rolled steel strip has a yield strength between 130-210 megapascals (MPa), a tensile strength greater than 260 MPa, a uniform elongation greater than 15%, a total elongation to failure greater than 30%, and a strain hardening exponent (n-value) greater than 0.2.

In some instances, the steel slab is hot rolled in the roughing treatment at temperatures between 1200-1300° C. In other instances, the steel slab is hot rolled in the roughing treatment at temperatures between 1220-1280° C. In still other instances, the steel slab is hot rolled in the roughing treatment at temperatures between 1230-1270° C.

The finishing treatment has an entry temperature between 1000-1080° C. and an exit temperature between 750-825° C. In some instances, the finishing treatment has an entry temperature between 1035-1065° C. and an exit temperature between 760-800° C.

The hot rolled strip produced from the finishing treatment can be coiled at temperatures between 640-750° C., and in some instances coiled at temperatures between 660-700° C. In addition, the hot rolled strip has a thickness between 1.5-6.5 millimeters (mm).

In one embodiment of the invention, the hot rolled strip is subjected to only air cooling while traveling across a run-out table between the finishing treatment and the coiling station. Stated differently, the hot rolled strip is not subjected to water or liquid cooling between the finishing treatment and the coiling station. In some instances, the hot rolled strip is only static air cooled on the run-out table between the finishing treatment and the coiling station, i.e. the hot rolled strip is not subjected to forced air cooling between the finishing treatment and the coiling station.

The coiled hot rolled strip can have a uniform elongation of at least 17.5% and a total elongation of at least 35%. In some instances, the coiled hot rolled strip has a uniform elongation of at least 20% and a total elongation of at least 38%.

The steel slab can have a carbon content of at least 0.025 wt % C. For example, the steel slab can have between 0.050-0.080 wt % C. The chemical composition of the steel slab can also meet a number of different chemical composition criteria, for example the Mn to S ratio (Mn/S) of the steel slab can be at least 15; the Ti and N content can obey the relationship −0.001 ≦(Ti−3.42N)≦0.002; the B and N content can obey the relationship −0.001≦(B−0.78N)≦0.002; and the Mn/S ratio can obey the relationship Mn/S >15.

The coiled hot rolled steel strip can be subsequently cold rolled to produce cold rolled sheet, followed by hot dip galvanizing or continuous annealing of the cold rolled sheet without an over-aging step. In addition, the inventive material and/or process can provide a bake hardenable steel with a yield strength between 180-210 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a temperature versus time graphical representation illustrating a process according to an embodiment of the present invention;

FIG. 2 is a graphical plot of 0.2% yield strength versus coil thickness for ferritic hot rolled carbon steel produced according to an embodiment of the present invention compared to the 0.2% yield strength of conventional austenitic hot rolled carbon steel;

FIG. 3 is a graphical plot of tensile strength versus coil thickness for ferritic hot rolled carbon steel produced according to an embodiment of the present invention compared to the tensile strength of conventional austenitic hot rolled carbon steel;

FIG. 4 is a graphical plot of percent elongation to fracture versus coil thickness for ferritic hot rolled carbon steel produced according to an embodiment of the present invention compared to the percent elongation to fracture of conventional austenitic hot rolled carbon steel; and

FIG. 5 is a graphical plot of strain hardening exponent (n-value) versus coil thickness for ferritic hot rolled carbon steel produced according to an embodiment of the present invention compared to n-values of conventional austenitic hot rolled carbon steel.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for producing ferritic hot rolled carbon steel that can be used for fabrication of components via deep draw stamping (DDS), cold rolling, and the like. As such, the present invention has use as a process for producing carbon steel sheet.

The inventive process uses or can use steels with a minimum carbon content in order to avoid cobbling of the material in the hot strip mill. In some instances, the minimum carbon content avoiding cobbling is 0.025 wt % in combination with suitable temperatures and set points in the finishing train. In addition, carbon contents between 0.050 to 0.080% can be processed to produce soft bake hardenable (BH) steels with a minimum 0.2% yield strength between 180 to 210 MPa and sufficient shelf time capability after a hot dip galvanizing process or a continuous annealing process without an over-aging section. It is appreciated that BH steels are known to those skilled in the art to be steels that exhibit or can exhibit a significant increase in strength, e.g. a 30 MPa increase, through a combination of work hardening, e.g. cold forming of steel sheet during production of a fender panel, followed by strain aging via a thermal treatment, e.g. paint baking of the fender panel after it has been painted.

The process includes hot rolling a steel slab having a chemical composition (in weight percent) within the range of 0.10 max C, 0.15-0.60 Mn, 0.20 max Si, 0.04 max Ti, 0.008 max V, 0.006 max Mo, 0.1 max Ni, 0.05 max Cr, 0.08 max Cu, 0.015 max S, 0.04 max P, 0.01 max N, 0.006 max B, 0.06 max Al, with a balance Fe and possible melting impurities. In one embodiment, the steel slab has a composition of 0.10 C, 0.15-0.60 Mn, 0.06 max Si, 0.009 max Ti, 0.008 max V, 0.006 Mo, 0.1 max Ni, 0.05 max Cr, 0.08 max Cu, 0.015 max S, 0.04 max P, 0.005 max N, 0.0015 max B, 0.03-0.017 Al with the balance Fe. In addition, the ratio of manganese to sulfur (Mn/S) is greater than 15, i.e., Mn/S >15.

In another embodiment, the steel slab has a chemical composition of 0.10 max C, 0.15-0.60 Mn, 0.04-0.20 Si, 0.01-0.04 Ti, 0.008 max V, 0.006 max Mo, 0.1 max Ni, 0.05 max Cr, 0.08 max Cu, 0.015 max S, 0.04 max P, 0.005 max N, 0.0015-0.0045 B, 0.08-0.5 Al with the balance Fe. In addition, the amount of Ti minus 3.42 times the amount of N is equal to a value between −0.001 to 0.002, i.e., −0.001 ≦(Ti−3.42N)≦0.002. Furthermore, the amount of B minus 0.78 times N is between −0.001 to 0.002, i.e. −0.001 ≦(B−0.78N)≦0.002. The steel slab can also have a Mn and S content such that Mn/S >15.

The slab thickness can be between 50 and 280 mm and is subjected to a soaking temperature in order to ensure that most if not all of the alloying elements are in solid solution. After soaking, the steel slab is subjected to a roughing treatment in which the material is hot rolled at a temperature or temperature range between 1100-1400° C. The roughing treatment produces a transfer bar which is subjected to a hot rolling finishing treatment. The entry temperature of the finishing treatment is between 900-1100° C. and the exit temperature of the finishing treatment is between 720-850° C. The finishing treatment produces hot rolled strip which is coiled at a temperature between 580-780° C.

The hot rolled strip has a thickness between 1.5 and 6.5 mm, a 0.2% yield strength of 130-210 megapascals (MPa), a tensile strength of greater than 260 MPa, and a total elongation of greater than 30%. The uniform elongation is greater than 15% and the strain hardening exponent ‘n’ is greater than 0.2. The microstructure of the material is ferritic and the steel exhibits excellent weldability. For the purposes of the present invention, the n-value is defined by the expression of the form σ=Kε^(n) where for an induced strain ε, the corresponding stress σ is the new yield strength of the material caused by the degree of cold working that has induced the strain ε. As such, and not being bound by theory, the greater the value of n for a material, the greater the degree of work hardening the material exhibits upon cold forming and thus the greater the yield strength and tensile strength of the material after being cold formed into a component.

Hot rolled strip produced by the inventive process is produced without an aging treatment, and is soft and ductile to the extent that it can be used for DDS. It is appreciated that the hot strip is soft and ductile due to a coarse and homogeneous ferrite grain size. The ferritic hot strip also has low aging sensitivity due to a low solute nitrogen content and/or generally fast aluminum nitride precipitation in the ferrite phase during rolling and coiling.

The hot strip is suitable for cold rolling and annealing with a corresponding high performance at the cold rolling mill in terms of productivity, less roll wear and less energy consumption compared to austenitic hot rolled strip. In addition, lower reheating temperatures are required during processing of the ferritic hot strip and thus less oxidation and energy consumption occurs in the reheating furnace. The less roll wear is due to reduced rolling temperatures and less intermediate roll changes. Finally, no cooling water is needed on the run-out table after the finishing treatment.

The process can include rough rolling of the steel in the austenite phase region for the material, finish rolling in the ferrite phase region and stress free recrystallization on the run-out table and during coiling. Such processing provides for a softer and more ductile material that can be used for DDS operations and/or cold rolling and annealing. Furthermore, it should be appreciated that the chemical composition of the steels is not restrained to the ELC or ULC steels taught by the prior art.

In order to provide a specific example of the present invention, but not limit the scope in any way, the following example is provided.

A steel slab having a chemical composition of 0.025-0.045 C, 0.32-0.4 Mn, 0.03 max Si, 0.008 max Ti, 0.008 max V, 0.05 Mo, 0.1 Ni, 0.05 Cr, 0.05 Cu, 0.01 S, 0.015 P, 0.005 N, 0.003-0.0045 B, 0.02-0.04 Al with the remainder Fe was soaked at an elevated temperature and subjected to a roughing treatment at temperatures between 1230-1270° C. Thereafter, the transfer bar produced from the roughing treatment was subjected to a finishing treatment that had an entry temperature between 1035-1065° C. and an exit temperature between 760-800° C. The finishing treatment produced hot rolled strip with a thickness between 3.0-3.2 mm that was coiled at temperatures ranging from 660-700° C. The material was also subjected to flattening elongation, also known as tension leveling, of between 0.3 and 0.7% at a pickle line. There was no cooling of the material after the finishing treatment and thus no cooling water was provided on the run-out table. For the purposes of the present disclosure, flattening elongation refers to stretching the hot rolled strip beyond its yield point and thereby providing a small amount of elongation and flattening of the product.

Samples of ferritic hot strip produced by the inventive process had a 0.2% yield strength between 130-210 megapascals, a tensile strength greater than 260 megapascals, and a total elongation of greater than 38%. Uniform elongation was greater than 20% and the strain hardening exponent was greater than 0.20. The microstructure was fully ferritic and the material exhibited excellent weldability.

Turning now to FIG. 1, a graphical representation of the process is shown via a temperature versus time plot. As shown in the figure, the material is soaked, e.g. at 1250° C., followed by a roughing treatment, then a finishing treatment and then coiling of the material. FIGS. 2, 3 and 4 illustrate yield strength, tensile strength, and total elongation, respectively, for samples of the ferritic hot strip having the above chemical composition range compared to conventional hot strip formed by prior art austenitic hot rolling. As shown by the figures, the ferritic hot strip is generally softer and more ductile than the conventional hot rolled material. In addition, FIG. 5 provides a comparison of the work hardening exponent ‘n’ for the two materials, and again, the ferritic hot rolled material disclosed herein exhibits superior work hardening compared to conventional austenitic hot rolled strip.

In view of the teaching presented herein, it is to be understood that numerous modifications and variations of the present invention will be readily apparent to those of skill in the art. The foregoing is illustrative of specific embodiments of the invention, but is not meant to be a limitation upon the practice thereof. As such, it is the claims, and all equivalents thereof, that define the scope of the invention. 

We claim:
 1. A process for manufacturing a ferritic hot rolled steel strip comprising: providing a steel slab having a chemical composition in weight percent within a range of 0.10 maximum (max) C, 0.15-0.60 Mn, 0.20 max Si, 0.04 max Ti, 0.008 max V, 0.006 max Mo, 0.1 max Ni, 0.05 max Cr, 0.08 max Cu, 0.015 max S, 0.04 max P, 0.01 max N, 0.006 max B, 0.06 max Al, balance Fe and melting impurities; soaking the steel slab within a temperature range between 1100 to 1400° C.; hot rolling the soaked steel slab in a roughing treatment at temperatures between 900 to 1400° C. and producing a transfer bar; hot rolling the transfer bar in a finishing treatment using a finishing treatment at entry temperatures between 900-1100° C. and exit temperatures between 720-850° C. and producing hot rolled strip; and coiling the hot rolled strip at a coiling station at temperatures between 580-780° C.; the coiled hot rolled steel strip having a yield strength between 130-210 MPa, a tensile strength greater than 260 MPa, a uniform elongation greater than 15%, a total elongation to failure greater than 30% and an n-value greater than 0.2.
 2. The process of claim 1, wherein the slab is hot rolled in the roughing treatment at temperatures between 1200-1300° C.
 3. The process of claim 2, wherein the slab is hot rolled in the roughing treatment at temperatures between 1220-1280° C.
 4. The process of claim 3, wherein the slab is hot rolled in the roughing treatment at temperatures between 1230-1270° C.
 5. The process of claim 1, wherein the finishing treatment has an entry temperature between 1000-1080° C. and an exit temperature between 750-825° C.
 6. The process of claim 5, wherein the finishing treatment has an entry temperature between 1035-1065° C. and an exit temperature between 760-800° C.
 7. The process of claim 1, wherein the hot rolled strip is coiled at temperatures between 640-750° C.
 8. The process of claim 7, wherein the hot rolled strip is coiled at temperatures between 660-700° C.
 9. The process of claim 1, wherein the hot rolled strip has a thickness between 1.5-6.5 mm.
 10. The process of claim 1, wherein the hot rolled strip is only air cooled on a run-out table between the finishing treatment and the coiling station.
 11. The process of claim 10, wherein the hot rolled strip is only static air cooled on the run-out table between the finishing treatment and the coiling station.
 12. The process of claim 1, wherein the coiled hot rolled strip has a uniform elongation of at least 17.5% and a total elongation of at least 35%.
 13. The process of claim 12, wherein the coiled hot rolled strip has a uniform elongation of at least 20% and a total elongation of at least 38%.
 14. The process of claim 1, wherein the steel slab has at least 0.025 wt % C.
 15. The process of claim 14, wherein the steel slab has between 0.050-0.080% C and is the coiled hot rolled steel strip is a bake hardenable steel with a yield strength between 180-210 MPa.
 16. The process of claim 15, further including the steps of cold rolling the hot rolled strip to produce cold rolled sheet followed by hot dip galvanizing or continuous annealing of the cold rolled sheet without an over-aging step.
 17. The process of claim 1, wherein the ratio of Mn to S ratio (Mn/S) in the steel slab is at least
 15. 18. The process of claim 1, wherein the Ti and N content in the steel slab obeys the relationship −0.001 ≦(Ti−3.42N)≦0.002.
 19. The process of claim 1, wherein the B and N content in the steel slab obeys the relationship −0.001≦(B−0.78N)≦0.002.
 20. The process of claim 1, wherein the Mn/S ratio in steel slab obeys the relationship Mn/S >15. 